1. Field of the Invention
The present invention relates to a quadrature oscillator, in particular to an oscillator for use in wireless communication devices having the advantages of low phase noise and low power dissipation.
2. Description of Related Arts
A quadrature oscillator is an oscillator that outputs with four different phases mutually separated by a quarter of a cycle, or 90 electrical degrees. This type of oscillator is generally used in the transceiver circuit of communication devices, such as cellular phones, wireless telephones, wireless networking devices, and all short-range blue-tooth communication devices. The transceiver circuits of these communication devices usually have a super heterodyne architecture, requiring a high precision filter for image rejection. However, it is difficult to produce transceiver ICs embedded with high precision filters. Therefore, the usual practice is to have a high precision filter soldered onto the transmitter circuit, thus posing a space constraint for circuit layout and making it difficult to reduce the related production costs.
To solve the above problems, direct conversion or low IF transceiver architecture is proposed to avoid the use of an image rejection filter. To support the above two architectures, transceivers have to rely on the oscillator outputs with precise phase quadrature. How to develop a high precision oscillator having quadrature output has become an interesting topic for discussion in the industry.
The above quadrature oscillator can be built using any one of the three methods mentioned below. The first method is to use two frequency dividers operated out-of-phase to produce the quadrature phase outputs. The second method is to use a conventional two-phase oscillator with a phase shifter such as a poly-phase filter to produce quadrature phase outputs. The third method is to use two cross-coupled two-phase oscillators, on which the present invention is based. The cross coupling of two two-phase oscillators forms a quadrature oscillator, as shown in FIG. 11, including:
two symmetrical oscillators (75) (76) being respectively connected by transistor pairs (M1, M2) (M3, M4) in a positive feedback structure to produce negative resistance;
two tank circuits (73) (74) being respectively connected to the above oscillators (75) (76) to produce positive resistance offsetting the negative resistance through the above oscillators (75) (76); and
two coupling circuits (71) (72) being respectively connected to the oscillators (75) (76) to produce quadrature phase outputs.
Under the above-mentioned architecture, one of the two oscillators (75) (76) has two outputs with phase shift of 0 and 180 electrical degrees, and the other outputs with phase shift of 90 and 270 degrees.
Since the oscillation frequency of the oscillator largely depends on the resonant frequency of the tank circuit and the parasitic capacitance in the active circuit, and the output amplitude is dependent on the Q of the tank circuit and the negative resistance in the active circuit, it is clear that the tank circuit plays an important role in this type of oscillator. However, as the Q of the tank circuit is largely determined by the manufacturing process of the semiconductor, designers can do little to alter its value, but it is possible to change the active circuit to improve the output characteristics of the oscillator using economical means.
For an oscillator, the characteristics of the active circuit can be analyzed through its linear characteristics in steady state. The negative resistance generated by per unit output current can be used as an indicator. In FIG. 11, the quadrature oscillator is formed by two cross-coupled symmetrical oscillators. Taking either one of the two for the present analysis will produce identical result. Accordingly, the small signal conductance of the oscillator at point V0 can be represented by:       G    2    =            -              g        m1              +                                        g            l1                    ⁢                      g            l3                          +                              g            l1                    ⁢                      g            m1                                                g          m3                +                  g          mb3                +                  g          l1                +                  g          l3                    
where
gm represents the transistor gate-to-source transconductance,
gl represents the small signal conductance across the drain and source, and
gmb represents the transistor bulk-to-source transconductance.
From the above-mentioned architecture of the quadrature oscillator, it is clear that the source terminals of transistor pairs (M5, M6) (M7, M8) in the coupling circuits (71) (72) are respectively connected in series to the drain terminals of transistor pairs (M1, M2) (M3, M4) in the two oscillators (75) (76). At the steady state operating point, the four output terminals are at the same potential V0=V180=V90=V270. For the transistor M1, the potential across its drain and source terminals is equal to VDS1=V270xe2x88x92VTH5, which is lower than the potential across the gate and source terminals, that means the transistor M1 is affected by the coupling transistor M5, causing the operating points of transistor M1 to shift toward the linear region and resulting in the decrease of transconductance (gm).
Under the pre-condition not to increase the operating current, it is necessary to increase the length/width of coupling transistor M5 such that the physical appearance of transistor M5 will be considerably larger than oscillator transistor M1, but this can cause proportional increase in parasitic capacitance that will decrease the oscillation frequency and the tunable frequency range.
From the foregoing, it is found that the operating point of the transistors in the conventional quadrature oscillator is seriously affected by the coupling transistors to shift toward the linear region, causing degradation of the signal conductance and the operating characteristics of the quadrature oscillator.
The main object of the present invention is to provide an oscillator that can effectively prevent shifting of the operating point of the oscillator toward the linear region in order to boost transistor transconductance, without degrading the oscillation frequency and the amplitude of the quadrature oscillator.
To this end, the architecture of the quadrature oscillator in accordance with the present invention comprises:
two symmetrical oscillators being connected by two cross-coupled transistors in a positive feedback structure to produce negative resistance;
two LC circuits being respectively connected to the above oscillators to produce positive resistance offsetting the negative resistance through the above oscillators; and
two coupling circuits being coupled to the above oscillators in order to produce outputs with phase quadrature.
The two coupling circuits are each formed by two identical transistors, where the drain terminals of the transistors are respectively connected to the source terminals of the oscillators, such that the operating point of the oscillator transistor is not affected by the coupling circuit and continues to work in the saturation region, thus reducing phase noise and maintaining the operating current with low power dissipation.
The features and structure of the present invention will be more clearly understood when taken in conjunction with the accompanying drawings.